Deep fryers are commercially used by restaurants, institutional kitchens, and fast food establishments for cooking a variety of food products, such as french fries, fish, fried chicken, and the like. The food product is cooked by totally immersing it within a vat or tank that is filled with heated oil or shortening. The oil may be heated using a flow of heated gas that is forced or drawn through a heat exchanger having flow passages that are located within or external to the cooking vat. A gas burner is provided to heat the gas circulating within the heat exchanger.
Heat is transferred in the vat predominantly by convection via the shortening. If the shortening is removed from the vat, the remaining air does not act as a sufficient heat transfer agent and the heat exchanger must rise to a significantly higher temperature to transfer the heat being fired into it by the less effective convection via the air, and by radiation. This condition is know as dry firing and results in accelerated failure of the heat exchanger in essentially all cases.
To detect this dryfiring, or overtemperature condition, which occurs when the shortening level in the vat drops, a temperature sensor can be placed in the vat which will trigger a shutoff device to deactivate the heating mechanism when the temperature in the vat reaches a predetermined maximum temperature. If the sensor is placed below the heat exchanger, a partial drain of the shortening could leave the top portion of the heat exchanger uncovered, and, therefore, exposed to excessive heat, while the sensor remains covered by shortening at a suitable temperature. In this case, the sensor would not trigger the heating mechanism to shut off, leading to failure of the top portion of the heat exchanger. Alternatively, the sensor may be placed above the heat exchanger in the area known as the cooking zone of the vat. A problem with locating the sensor in the cooking zone is that it may interfere with the food product and baskets holding food product which are placed in the cooking zone. A sensor located in the cooking zone above the heat exchanger also generally has a poor radiative heat transfer form factor, or angle factor, that is, the fraction of diffuse radiant energy falling on the sensor, relative to the heat exchanger. This results in a poor transfer of heat from the heat exchanger to the sensor. Consequently, it is difficult to get the sensor hot enough to trip and deactivate the heating mechanism under dry fire conditions, while keeping it cool enough to avoid deactivating the heating mechanism during suitable operating conditions. A sensor located at the top of the flow passages of the heat exchanger would necessarily be at the same height as the welds joining the flow passages to the vat, and would also get hot more slowly than the welds since the welds are in close proximity to, or in contact with, the flame from the burner.
It is an object of the present invention to provide a deep fryer with a dry fire limiting device which reduces or wholly overcomes some or all of the aforesaid difficulties inherent in prior known devices. Particular objects and advantages of the invention will be apparent to those skilled in the art, that is, those who are knowledgeable or experienced in this field of technology, in view of the following disclosure of the invention and detailed description of certain preferred embodiments.